The burgeoning antibiotic resistance crisis is an imminent threat to the practice of modern medicine and global public health. In particular, nosocomial infections caused by multi-drug resistant (MDR) Gram- negative rods (GNRs) such as carbapenem-resistant strains of Klebsiella pneumoniae (Kp), Escherichia coli (Eco), Enterobacter cloacae (Ecl), Proteus mirabilis (Pm), Acinetobacter baumannii (Ab) and Pseudomonas aeruginosa (Pa) are spreading through horizontal transfer and clonal expansion with limited therapeutic options and high attendant morbidity and mortality. We are challenging the current standard MIC/MBC testing in bacteriologic media, which is totally agnostic to the host immune system. Even before the first diagnostic encounter with a physician, a patient?s infection is already being combatted by numerous endogenous immune effectors, including antimicrobial peptides, serum complement and phagocytic cells. While tremendous effort has been extended to identify and exploit synergy between different classes of pharmaceutical antibiotics, very little work has been conducted to study the way pharmaceutical antibiotics interact with endogenous antimicrobial defenses. We have documented unexpected activity of azithromycin (AZM), the most commonly prescribed antibiotic in the US, to sensitize highly MDR strains of Kp, Ab and Pa to innate immune killing ? even though this drug is not even included in the AST panel when such isolates are encountered. Second, we have documented unexpected activity of ?-lactamase inhibitors (BLIs) such as tazobactam (TAZ) and avibactam (AVI) to sensitize highly MDR strains of Kp and Ab to innate immune killing ? even though these agents are never contemplated to have direct action beyond blocking ?- lactamase enzymes. In the present proposal, we will identify synergistic interactions wherein AZM or BLIs sensitize MDR GNRs to killing by host AMPs, serum complement or phagocytic cells using checkerboard and kinetic killing assays, assays of bacterial physiology including bacterial cytological profiling (BCP), and studies of sensitization to complement, platelets, and phagocytes. Then using Ab and Kp as model MDR GNRs, we will translate innate immune sensitization by AZM and BLIs to effective therapeutic options in murine models of pneumonia, sepsis and UTI, using pharmacologically or genetically targeted mice to have defects in innate immunity (cathelicidin, complement, neutrophils) to identify key synergistic host factors. Careful attention is paid to provide alternative methods for mechanism of action determination (e.g. metabolic precursor incorporation, transmission electron microscopy, whole genome sequencing of induced resistant variants) and to assess other potential immunomodulatory effects of AZM and BLIs on cytokine signaling. As our earlier work with MDR Gram+ pathogens (MRSA, VRE) and ?-lactam sensitization to innate immunity was corroborated in clinical series and influenced treatment guidelines, we foresee immediate translational impact of the current investigations on MDR GNR pathogens.